Polyamide-imides comprising the reaction products of tribasic acid anhydrides and a diisocyanate are known to form useful insulating coatings, e.g., for electrical conductors. See, e.g., Nakano and Koyama, U.S. Pat. No. 3,541,038. See also Terney et al., J. Polymer Science A-1, Vol. 8, pages 683-692 (1970). Applicant's copending application Ser. No. 137,991, filed Apr. 7, 1980, discloses that the reaction products of bisetheranhydrides and a diisocyanate are resinous products also having such utility. Moreover, applicant and her coworker Zamek in a copending application, Ser. No. 138,198, filed Apr. 7, 1980, disclose that reaction products of tribasic acid anhydrides, bisetheranhydrides and diisocyanates are resinous reaction products with excellent high thermal resistance and electric insulation properties. The disclosures of the foregoing article, patent and applications are incorporated herein by reference.
As a result of numerous tests and experiments, certain deficiencies have been noted in wire coating compositions made following disclosures in the prior art. For one thing, the reactants appear to require somewhat longer times then the expected 2-4 hours to reach the great increase in viscosity suggested to be necessary. This has been overcome by applicant's discovery that oxyanion or amine compounds such as 2-methylimidazole, act as efficient catalysts in making the polyamide-imides by the earlier processes. In the earlier processes, too, the high molecular weight resins usually only produce enamel compositions with relatively low solids content, e.g., of below 30% by weight and usually of 21 to 25%. Such compositions require more passes through the coating machines to produce layers of suitable thickness. Moreover--inexplicably--if higher solids enamels are prepared by the prior art processes, the enamels have definitely poor shelf life. The prior art resins also are difficult, if not impossible, to dissolve in conventional solvents, i.e., those having a significant content of hydrocarbon, and this too is a disadvantage, because of economic considerations, but especially because of stability problems.
It has now been discovered that uniquely useful reaction products comprising polyamide-imide resins can be formed from the reaction of a tribasic acid anhydride and a diisocyanate if a catalyst is used and the reaction is short-stopped at a moderate viscosity at a high solids content. Moreover, even though they can be diluted to a low solids range, if the resins are applied from high solids solutions in an organic solvent they can be used as such for top coats. Surprisingly good shelf lives are obtained. The products find use as insulating coatings for electrical conductors, e.g., magnet wire and magnet strip, because they have good runnability and provide excellent electrical insulation properties. In addition, the high solids content enamels or this invention show less tendency to develop haze, which is another indication of their superiority.
In preferred features, the resin will be prepared in the presence of a catalytic amount, from a trace to about 10 mole percent (based on the tribasic acid anhydride) of an oxyanion compound, or an amine, secondary or tertiary, e.g., trimethyl amine, triethyl amine, di-n-butyl amine, pyridine, 2-phenylimidazole, imidazole, water, organotin compounds, and the like, preferably 2-methylimidazole. The term oxyanion defines a family of compounds known to catalyze the reaction of isocyanates.
The tribasic anhydride component, is described in the above-mentioned Nakano et al. U.S. Pat. No. 3,541,038. Suitable are aromatic, aliphatic and alicyclic tribasic acid anhydrides, such as, for example, trimellitic anhydride, hemimellitic anhydride and aconitic anhydride. Trimellitic anhydride is preferred.
The organic diisocyanates can be prepared in ways known to those skilled in this art, and they are also commercially available. Any of those described in U.S. Pat. No. 3,541,038 can be used. For example, C.sub.4 -C.sub.24 alkyl, cycloalkyl or aryl diisocyanates are useful, e.g., hexamethylene diisocyanate, tetramethylene diisocyanate, and the like, cyclohexane diisocyanate, cyclopentane diisocyanate, and the like, diphenyl oxide diisocyanate, diphenylmethane diisocyanate, tolylene diisocyanate, phenylene diisocyanate, xylenyl diisocyanate, diphenylsulfone diisocyanate, naphthalene diisocyanate, and the like can be used. It is seen from the foregoing illustrations that the C.sub.4 -C.sub.24 carbon chains can be interrupted by hetero atoms such as oxygen, nitrogen, sulfur, and the like.
Preferred are diphenylmethane diisocyanate, diphenyl oxide diisocyanate and hexamethylene diisocyanate.
The polymer is prepared from the reaction of tribasic anhydride and the corresponding diisocyanate (DI) in the presence of the catalyst, in an organic solvent, such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAC), or mixtures containing an aromatic hydrocarbon, e.g., of 6 to 40 carbon atoms, e.g., xylene, or a proprietory aromatic hydrocarbon solvent, e.g., Solvesso 100, Mixed solvents can comprise NMP-DMAC, or NMP-xylene. As has been mentioned, hydrocarbon alone will not function as the inert solvent.
The preferred reaction pathway uses trimellitic anhydride, and is as follows: ##STR1## where R is divalent alkylene, cycloalkylene, arylene, of from 4 to 24 carbon atoms, or a mixture thereof.
The optimum mole ratio of DI (analog) to TMA is 0.95 to 1.05, preferably 0.99-1.01 to 1.00, and the catalyst is 5-10 mole %, based on dianhydride. To make a coating composition the resin can be prepared in the organic solvent, e.g., NMP, NMP-DMAC, NMP-xylene or Solvesso 100, etc., or the resin can be isolated, then redissolved in such solvents, or in methylene chloride, dimethyl formamide, and the like. Cresylic acid and/or phenol cannot be used.
Preparative methods are used which are not conventional in the sense set forth in the above-mentioned U.S. Pat. No. 3,541,038.
For example, instead of heating the reactants at 60.degree. to 200.degree. C., in the absence of a catalyst, a catalytic amount of an oxyanion or amine compound, e.g., 2-methylimidazole, is used. Instead of heating until the "viscosity is greatly increased" (Examples 1, 2, 3 of Nakano) or to a constant viscosity, i.e., the highest which can be reached, it is essential to short-stop the reaction, e.g., by cooling and/or adding more solvent and/or a terminating agent, at a moderately low viscosity, e.g., a Gardner viscosity no higher then Z3 to Z6, at a solids content of between 31 and 45% by weight. Also contemplated to control the viscosity is to use a monofunctional compound, i.e. isocyanate, anhydride, or carboxylic acid, or active hydrogen compound, as a viscosity controlling agent or terminating compound. This can be added prior to heating, and in this case tends to slow the viscosity build rate, and permit more latitude in the time allowed to terminate, e.g., by cooling and/or adding solvent. Alternately, the terminating agent can be added when the viscosity reaches the desired level, and this not only prevents immediate increases, but tends to prevent any increase in storage. Suitable terminating agents are numerous, for example, there can be used phenyl, isocyanate, benzoic acid, phenol, t-butyl alcohol, diethylene glycol monomethyl ether (DM), and the like. The latter two are preferred.
In any case, the viscosity end point is important to observe. To fail to do so is to risk the likelihood of producing high solids enamels which are unstable.
After the resin has been made, if desired, it can be adjusted to a solids content of no less than 30% by weight with one of the solvents disclosed herein, but preferably a mixture of such solvents. In any case, the preferred mixture will comprise N-methylpyrrolidone and an aromatic hydrocarbon solvent Solvesso 100, well known for making coating compositions. Conveniently, the organic solvent mixture can also be of the same composition employed as the inert reaction solvent. The mixed solvent can vary in ratio, but most preferably, from about 2 to 4 parts by weight of N-methylpyrrolidone per 1 part by weight of aromatic hydrocarbon will be used. In optimum cases, the final viscosity, measured on a Brookfield apparatus at 25.degree. C. with Number 3 spindle at 10 and 20 rpm will be in the range of 2800-4000 cps at 31 to 33 percent solids. This corresponds to a Gardner viscosity of Z3/4 to Z21/4. In the optimum composition, also at this solids level, the acid number will be no less than about 4 and within the range of 4-8.
Such enamels can be used as a top coat over a polyester or a polyesterimide base coat at any solids content, or as a sole coat at solids contents below about 32% by weight.
In accord with conventional practices, other additives may be formulated into the compositions, such as, without limitation, minor proportions of aliphatic amino compounds, conventional phenolic resins, titanate esters, blocked polyisocyanates, and the like.